Membrane fusion mediates many important biological events including fertilization, muscle development, synaptic transmission, and infections by enveloped viruses. Viral fusion reactions are mediated by specific membrane glycoproteins. Similar proteins most likely catalyze cellular fusion events. The long term objective of this project is to characterize the molecular mechanisms of viral and cellular membrane fusion proteins. The best characterized viral fusion protein is the influenza hemagglutin (HA). Under fusion-triggering conditions, the HA undergoes a conformational change which exposes a previously buried, apolar, and highly conserved "fusion peptide". Insertion of this peptide into the target membrane initiates the fusion reaction. The first major goal of this project is to determine if the general features of this mechanism are shared by other viral fusion proteins. The second major goal is to determine if an important cellular fusion process is mediated by an endogenous fusion protein. The specific aims for the proposed period of support are, therefore: (1) to compare the fusion mechanism of the env glycoprotein of Rous Sarcoma Virus with that of the influenza HA; and (2) to identify and characterize a myoblast fusion protein Biochemical, immunological, and molecular biological techniques will be employed. The virus project will involve analyzing the "fusion-inducing" conformational change in the env glycoprotein using anti-peptide antibodies, characterizing the interaction of env and its fusion peptide with target membranes, and analyzing (by site-specific mutagenesis) the specific amino acid requirements of the fusion peptide. The muscle project will involve the generation of antibodies that specifically inhibit the fusion of C2 mouse myoblasts. The medical relevance of this project is twofold. Firstly, since membrane fusion is a critical early step in the infectious cycle of viruses such as influenza and the human immunonodeficiency viruses, these studies should provide a basis for rational anti- viral strategies. Secondly, since membrane fusion is required for the development of skeletal muscle cells, these studies are relevant to our understanding and treatment of a variety of muscle cell disorders.